cluster

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A cluster is a range of simultaneously sounding pitches that
may change over time. The set of available pitches to apply usually
depends on the acoustic source. Thus, in piano music, a cluster
typically consists of a continuous range of the semitones as provided
by the piano's fixed set of a chromatic scale. In choral music, each
singer of the choir typically may sing an arbitrary pitch within the
cluster's range that is not bound to any diatonic, chromatic or other
scale. In electronic music, a cluster (theoretically) may even cover
a continuous range of pitches, thus resulting in colored noise, such
as pink noise.

Clusters can be denoted in the context of ordinary staff notation by
engraving simple geometrical shapes that replace ordinary notation of
notes. Ordinary notes as musical events specify starting time and
duration of pitches; however, the duration of a note is expressed by
the shape of the note head rather than by the horizontal graphical
extent of the note symbol. In contrast, the shape of a cluster
geometrically describes the development of a range of pitches
(vertical extent) over time (horizontal extent). Still, the
geometrical shape of a cluster covers the area in which any single
pitch contained in the cluster would be notated as an ordinary note.